Physics. — ‘“Hivtinction by a Blackened Photographic Plate as Function 
of Wavelength, Quantity of Silver, and Size of the Grains”. 
By Avra. Drumens. (Communicated by Prof. W. H. Junius). 
(Communicated at the meeting of Sept. 25, 1920). 
Introduction. For the right understanding of the photographic process 
knowledge of the final product: the blackened photographic plate, 
is indispensable. In this communication we shall give an account of 
a research of the blackened photographic plate. 
We have set ourselves the task to investigate the blackening * for 
different wave-lengths, to measure the quantity of silver present in 
the examined plates per unit of area, and we have further deter- 
mined the size of the silver grains ’). 
The extinction of a photographic plate may be compared to that 
of a fixed colloidal solution, e.g. milk-glass or ruby-glass, where 
also small particles are steeped in another medium. 
The quantities mentioned have been measured, because the extinc- 
tion of a blackened photographic plate can be theoretically under- 
stood by considering it as the result of the action of a great number 
of irregularly distributed grains of silver, steeped in gelatine. 
When the distribution and the nature of the silver in the plate 
is known, the action on radiation of given wave-lengths can be 
derived, and inversely something about the nature of the silver in 
the plate can be inferred from this action for given radiation on 
grains of given size. 
In this communication we shall not enter further into the theo- 
retical considerations, but reserve them for a following communication. 
We now confine ourselves to the description of the experimental 
methods used and of the results obtained by them. 
') By blackening is understood Brigg’s logarithm of “2 in which J), resp. J, 
is the intensity of the light that has traversed an unblackened, resp. blackened 
part of the plate. : 
The name “blackening” is not appropriate, because the photographic plate 
is not black, 
Hence it would be better to follow R. LUTHER (Zeitschr. f. phys. Chem. 
1900) and speak of “the extinction” of the photographic plate. 
*) Besides the extinction and silver content of a collargolsolution was, 
examined. 
